feat(allocator): add Vec2

[Dunqing]

related: #9706

These two files are copied from Bumplo's vec and raw_vec, they will be used to replace allocator_api2's Vec later PRs.

[Dunqing]

• feat(allocator): remove drop operations from Vec2 #9679
• feat(allocator): replace allocator_ap2's Vec with Vec2 #9656
• feat(allocator): add Vec2::retain_mut method #9655
• feat(allocator): connect Vec2 module and make it compile #9647
• feat(allocator): add Vec2 #9646 👈 (View in Graphite)
• main

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[codspeed-hq]

CodSpeed Performance Report

Merging #9646 will not alter performance

_{Comparing 03-10-feat_allocator_add_vec2 (3d4400c) with main (f9580d8)}

Summary

✅ 32 untouched benchmarks
🆕 1 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	linter[checker.ts]	N/A	3 s	N/A

[overlookmotel]

I double-checked these 2 files are copied completely verbatim from bumpalo. Only differences are the addition of comments at top of files saying where the code was copied from, and running rustfmt on them.

Merging just this one for now. Would like to look into why lifetime changes are required in #9656 before merging the rest of the stack.

[overlookmotel]

Merge activity

• Mar 12, 4:52 AM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Mar 12, 4:52 AM EDT: A user added this pull request to the Graphite merge queue.
• Mar 12, 4:57 AM EDT: A user merged this pull request with the Graphite merge queue.